This invention relates to a process for the preparation of low molecular weight 4,4'-diphenylmethane-uretdione-diisocyanates corresponding to the formula: ##STR2## in which n represents 0 to 1, preferably 0 to 0.5.
Aromatic uretdione-diisocyanates ("dimeric" diisocyanates) and processes for the production thereof are known. See "Kunststoff-Handbuch", Volume VII, Polyurethane, published by Vieweg-Hochtlen, Carl-Hanser-Verlag, Munich, 1966.
Materials which have been described as dimerization catalysts useful in such processes include: trialkylphosphines (J. Org. Chem.8, 23 (1943)); aromatic-aliphatic tertiary phosphines; alkyl-diaryl phosphines (German Auslegeschrift No. 2,452,390); tri- or tetra- substituted pyridines (British Pat. No. 821,158); trialkyl phosphites (German Auslegeschrift No. 2,349,726) and phosphorous acid-tris-dialkylamides (U.S. Pat. No. 3,290,288). In some cases, the dimerization reaction to form the uretdione may take place in bulk. However, solvents which are inert to NCO groups are generally used. Examples of solvents described as suitable for such processes include: benzene, toluene, xylene, chlorobenzene, nitrobenzene, acetone, methylethylketone, acetic ester, dioxane, tetrahydrofuran, aliphatic hydrocarbons, dimethylformamide and methylene chloride. No distinction between these solvents (with respect to polarity or dissolving power) has been made in the literature.
The fact that undesirable by-products (for example, isocyanurate) may be produced during the dimerization of aromatic isocyanates where reactive dimerization catalysts are used was recognized in U.S. Pat. No. 2,683,144. To minimize formation of such by-products, it is recommended that dimerization be interrupted when the required dimerization stage has been reached. Alkylation agents (e.g., cyclohexane sulphonic acid methylester or benzyl chloride) are used as inhibitors (stoppers) for the dimerization catalyst.
The literature teaches that aromatic diisocyanates which are substituted by other radicals R (for example, alkyl groups) in one or both ortho-positions relative to an NCO group, produce uretdione-diisocyanates having double the molecular weight of the starting compounds. An example of one such compound is the dimeric toluylene diisocyanate: ##STR3## This teaching has been extended in the patent literature to aromatic diisocyanates in which the ortho-position (relative to the aromatically bound NCO group) has only H atoms. The conventional term for such uretdione-diisocyanates is a "diphenylmethane-diisocyanate-dimer". The diphenylmethane-diisocyanate-dimer based on 4,4'-diphenylmethane-diisocyanate was believed to be generally represented by the formula: ##STR4##
German Auslegeschrift No. 1,445,721 (U.S. Pat. No. 3,290,288), however, teaches that in addition to dimeric uretdiones during the dimerization of diphenylmethane-diisocyanate, tri-, tetra- and pentameric uretdiones are produced. These compounds are represented by the formula: ##STR5## in which n=0 to 5.
German Auslegeschrift No. 2,419,968 discloses that the dimerized diphenylmethane-diisocyanates described in the literature may be used in the synthesis of high molecular weight polyurethanes having uretdione groups by a reaction in highly-polar solvents such as DMF. However, as the degree of polydimerization increases (n slightly greater than 2.5), the reacvitity with H-acid compounds decreases due to increasingly difficult solubility of the uretdione-diisocyanates. Consequently, such higher molecular weight oligomers are not suitable for many polyurethane reactions, particularly those which are carried out in the absence of solvent or at a temperature below the melting points of the uretdione-diisocyanates.
The significance of the solvent present during the dimerization reaction has not been discussed very often in the patent literature. More particularly, the influence of the solvent during the dimerization of aromatic diisocyanates in which the NCO group is flanked only by H-atoms in the ortho-positions has not been specifically considered. Some inert solvents which are described in the literature as suitable for the dimerization of aromatic isocyanates have been found to be useless in the dimerization of diphenylmethane-diisocyanate because high molecular weight MDI dimer mixtures (in which n is much greater than 2.5) are produced. Such high molecular weight mixtures are generally not suitable for other polyurethane reactions (see Example 1).
It has also been found that poly-dimers are produced almost exclusively in very highly polar solvents (such as in dimethylformamide or dimethylacetamide). Such poly-dimers are different in reactivity and convertibility from the low molecular weight compounds of the present invention.